Composite

ABSTRACT

A composite comprising a structural component and a matrix component, the structural component comprising structural fibers and a toughening additive comprising non structural fibers of a first thermoplastic material and the matrix component comprising a second thermoplastic material. The structural component is a fabric formed from the structural fibers and the non structural thermoplastic fibers, the fabric comprising non structural thermoplastic fibers which are in fibre form in the final composite. The first and second thermoplastic materials differ as to their molecular weight or are dissimilar.

The present invention relates to a composite and a method of compositemanufacture.

Composite materials generally comprise an array of reinforcing fibres ina resin matrix. The present global industries which utilise compositestructures, for example the aerospace industry, predominantly useconventional unidirectional and fabric-based prepregs. Such prepregs areformed by making a unidirectional roving of reinforcing fibres, drawingthe fibres through a bath of molten resin material and then drying theresin. The prepreg is then formed into a desired shape, loaded into amoult which is closed and heated to cure the resin.

Over the last five to seven years in an alternative technology formanufacturing composite parts has emerged which is generally termedliquid composite moulding. In liquid composite moulding, a dry fibrousreinforcement is loaded into a mould or tool and the resin is injectedor infused into the fibres and cured.

The reinforcement is termed a “preform” which term is well known tothose skilled in the art of composites as indicating an assembly of dryfibres that constitutes the reinforcement component of a composite in aform suitable for use in a liquid composite moulding process. A preformis typically an assembly of various textile forms such as fabrics,braids or mats, tailored or shaped as necessary, and is assembled as aspecific operation prior to being placed into or on the mould tool.

Liquid composite moulding technologies, such as the RTM (resin transfermoulding), SCRIMP (composite resin injection moulding or vacuuminfusion) methods are perceived by many to be the solution to theproblem of making composite parts in a number of intractable situations,such as large aerospace primary structures and high volume structuralautomotive components. The benefits that liquid composite mouldingtechnologies are perceived to offer over conventional prepregs arereduced scrap and lay-up time, non-dependence upon drape and increasedshelf life properties.

However, liquid composite moulding does possess its own problems,particularly, when the end use applications require high toughness andwhere control of curing cycle time is critical.

Structural parts require a high degree of toughness for mostapplications and this is especially true of aerospace primarycomponents. The solution to introducing high toughness in an aerospacegrade composite has traditionally been to toughen the matrix—usually bythe introduction of a second phase additive such as a thermoplasticpolymer to the base epoxy resin matrix.

Various approaches have been employed for the addition of athermoplastic material into the resin. The thermoplastic may be blendedwith the unreactive thermoset resin at elevated temperatures to producea single phase, unreacted melt. A limitation of this approach is thelevel of thermoplastic that can be added to enhance toughness. Highmolecular weight thermoplastics are used but as these dissolve into theresin, the viscosity of the blend rises steeply. However the very natureof the injection process of the resin into the reinforcing fibresrequires that the resins rheological properties, viscosity andelasticity are such as to allow infiltration of the resin throughout thefabric preform. This is essential if the resulting composite structureis to be free of voids and long injection times and high injectiontemperatures are to be avoided. Conventional toughened epoxies areextremely viscous systems which means that high pressures and massivetools are required with the necessity of heating the resins anddifficulties in matching curing time and injection-fill cycles.

Thermoplastic may also be added in the form of a continuous solid filmwhich is placed between two layers of fibre. In such processes thethermoplastic layer is generally known as the Interleaf layer. A processof this type is disclosed in European Patent Application No. 0327142which describes a composite which comprises a solid continuous layer ofa thermoplastic material placed between two layers of fibre impregnatedwith thermosetting resin. On heating the thermosetting layers and theinterleaf layers remain as discrete layers.

A problem with the interleaf approach is that the solid thermoplasticfilm does not dissolve into the resin during the heat processing stage.As a result, although the final composite may show the desired increasein toughness, there is a weak resin-thermoplastic interface. The weakinterface between the interlayer and matrix can cause poor resistance tocracking between plies especially when exposed to a moist environment.

Thermoplastic material may also be introduced in a powdered form. Anexample of this technique is disclosed in European Patent ApplicationNo. 0274899 where the thermoplastic material is either added to theresin before the prepreg is prepared or sprinkled onto the prepregsurface.

The use of powders presents a problem in that it is difficult to ensurethat an even distribution of powder is supplied to the resin. There istherefore an uneven loading of the thermoplastic material with theresult that the composite will have regions of different toughnesses.Furthermore, incorporation of powdered thermoplastic material in theresin is not suitable for liquid composite moulding techniques becausethe viscosity of the resin is increased when the particles are added toit according to standard Newtonian theory with all the consequentdisadvantages as discussed above.

Whether the powdered thermoplastic is added to the resin or to theprepreg, the amount which can be incorporated is limited. Thus, so toois the toughening effect and, in general, to achieve a reasonableimprovement in toughness, expensive structural thermoplastics have to beemployed.

It has been proposed, in Japanese Patent Application 6-33329, to includethermoplastic in the form of fibres. The Application discloses areinforcement fibre mix comprising 99.80% by weight of carbon fibres orgraphite fibres and 1-20% by weight of thermoplastic resin. Thisapproach is disclosed solely as useful in a classic prepreg technique.

A good composite is one having a combination of physical propertiesparticularly suited to a specific application. The physical propertiesof the composite product are determined by, amongst other things, thephysical properties of the solidified resin matrix material and thestructural material, and the uniformity of distribution of the matrixmaterial and the structural material in the composite. Best results areachieved where the matrix material is intimately in contact with all ofthe structural material.

It is therefore desirable that the resin matrix material is of such aconsistency (viscosity) that it covers (wets) all of the structuralmaterial and, if necessary, fills the interstices formed in thestructural material. Uniform wetting is particularly difficult toachieve where the structural material is of complex structure, forexample where it is a preform, or where the ratio of the matrix materialto support is particularly low.

The viscosity of the matrix material is affected by the number and typesof additives. There therefore arises the problem that, although a liquidor a gel matrix material, comprising one or more additives may possesssuitable physical properties when solidified, the viscosity of theliquid or gel matrix material may be too high to facilitate its evendistribution around the support material, particularly where the supportis complex. This results in a composite product lacking the physicalcharacteristics expected.

Normally to achieve a good combination of properties a compositematerial will consist of a number of constituents. Typically for anaerospace grade prepreg there will be a high performance reinforcementpreform combined with a complex polymeric resin matrix mix. This matrixmix normally consists of a thermosetting epoxy resin blended withvarious additives. These latter additives enhance the toughness of thebasic resin. Such systems have complex flow characteristics and whilstthey can be easily combined with fibres in a prepreg form, their use inother manufacturing techniques is limited. As for instance an attempt touse such a complex resin in an injection or resin transfer process in acomplex fibre preform may result in the filtering out of additives and anon uniform product.

There is therefore a need for a method of composite manufacture whichovercomes the above mentioned problems particularly for large complexstructures.

EPA 0539 996 describes a fabric for lamination or moulding comprisingreinforcing fibres such as carbon fibres and yarns of high and lowmolecular weight polyether either ketone resin. On application of heat,the low molecular weight yarns melt and enter the gaps between thefibres and the high molecular weight yarns.

In accordance with a first aspect of the present invention there isprovided a composite comprising a first structural component and asecond component, the structural component comprising structural fibresand a toughening additive comprising non structural fibres of a firstthermoplastic material and the second component comprising a secondthermoplastic material, wherein the structural component is a fabricformed from the structural fibres and the non structural thermoplasticfibres, wherein the fabric comprises non structural thermoplastic fibreswhich are in fibre form in the final composite, and wherein the firstand second thermoplastic materials are different, characterised in thatthe second component is a matrix component formed by injecting thestructural component with a liquid resin comprising the secondthermoplastic material.

The term “structural fibre” as used herein refers to fibres which add tothe strength of the ultimate composite such as glass or carbon fibresand which therefore have a modulus of elasticity greater than 50 GPa.

The term “non-structural fibre” as used herein refers to fibres whichare not provided for increasing the strength of the ultimate compositeas they have a modulus of elasticity less than 40 GPa. Thus knownstrengthening fibres formed from materials such as Kevlar are notnon-structural fibres within the terms of the present Application.

The composite uses a thermoplastic resin as the matrix. Thethermoplastic material may be expected to provide good chemicalresistance and a degree of toughness in the final part. However, inorder to achieve low viscosity which will be desirable if thethermoplastic resin is to be injectable, it may be necessary to reducethe molecular weight of the resin. Toughness of a thermoplastic isclosely linked to the molecular weight such that a decrease in molecularweight will result in a decrease in toughness. It is therefore proposedthat in addition to the use of a thermoplastic matrix, the composite isfurther toughened by the incorporation of thermoplastic fibres into thefibre preform.

Alternatively considered, the form of the reinforcement componentenables a reduction in the toughening to be provided by the matrix resinthus allowing use of low viscosity systems. In other words, by providingfor toughening of the composite by the fibres, the molecular weight ofthe thermoplastic resin can be made lower and so it may have a lowerviscosity. This makes impregnation of large parts feasible with sensiblepressures, lightweight low cost tooling and manageable cycle times.Furthermore a significantly greater amount of toughening additive can beincluded without compromising any of the processability aspects ofliquid composite moulding techniques.

The thermoplastic fibres may be produced from a similar thermoplasticmaterial to that of the matrix but with a higher molecular weight tointroduce toughness. Alternatively, the fibres may be produced from adissimilar thermoplastic material.

Combinations of thermoplastic fibres may also be used in order toachieve a mix of optimum properties. The properties of the compositewill be dependent on the mechanical properties of the matrix, theadditional thermoplastic fibres and the interfacial bonding between allcomponents.

By toughening is meant the ability to absorb fracture, which maymanifest itself in the ability to absorb impact. Such ability may bemeasured by suitable impact testing methods which will be known to theskilled man. Thermoplastic polymers are known to increase the ability toabsorb impact energy in structural composites. By suitable formation ofthe fabric they may be dispersed throughout the final composite to givehomogenous impact resistance.

In accordance with another aspect of the invention, there is provided amethod of making a composite comprising forming a fabric from structuralfibres and non structural fibres of a first thermoplastic material toprovide a structural component, injecting a liquid resin comprising asecond thermoplastic material into the structural component to provide amatrix component and setting the matrix component, wherein the first andsecond thermoplastic materials are different and wherein the liquidresin is injected at a temperature such that the final compositeincludes non structural thermoplastic fibres in fibre form.

The hybrid approach to the production of preforms for liquid compositemoulding involves integrating the mechanism that provides tougheninginto the novel textile preforms rather than being an additive in athermosetting resin. The net result of this is that the improvedproperties are achieved without compromising the manufacturability ofthe systems. This also has manufacturing benefits and in addition toproviding a tougher part it also simplifies the manufacturing processand allows the potential for faster manufacturing make span times andhence greater tool utilisation. This has the further benefit ofpotentially reducing the most expensive element of a new compositeprogramme: the up front investment required to meet rate production andhence provides the potential for lower cost entry into a new productprogramme.

Preferably, the toughening additive is a thermoplastic material whoselatent heat of melting may absorb a proportion of the heat of the resinbut which, upon completion of setting, reverts to its solid form withoutloss of toughening capacity. Alternatively, the thermoplastic resin andthermoplastic additive may be selected to allow absorption of some ofthe energy of setting in melting or phase change of the additive.

Injection of low viscosity resins can decrease the injection-fill partof the processing cycle. However, it is also desirable to decrease theremaining cycle time. By using very hot low viscosity thermoplasticresins, the injection-fill part can be accelerated but the risk is thegeneration of a long cooling cycle—particularly in thick parts, and alsoexcessive heating, again particularly in thick parts, which could leadto a degraded, distorted or damaged final part.

A very rapid cycle can be affected without risking excessive cool-outtimes if semi-crystalline thermoplastic fibres are used as thetoughening additive. Heat from the cooling of the thermoplastic matrixcan be used to generate crystalline melting with the fibres. The latentheat of crystalline melting will absorb the excess energy therebyaccelerating the cooling cycle and ensuring that it happens at a uniformrate within the bulk of the product thereby eliminating the potentialfor distortions to occur within the part. The selection of tougheningfibres with appropriate crystalline melting temperature allows the cycletime to be minimised without risk of composite damage. Thesemi-crystalline fibres themselves will simply revert to their originalcondition on cooling and the process will not affect the ultimatetoughness of the parts.

Preferred toughening additives include:—polypropylene, nylon 6, 6,styrene-butadiene, butadiene, polyether imide, polyethylketone, PET,polyether sulphone.

Preferably, the percentage by volume of the toughening additive in thefinal composite is more than 2%, more preferably, more than 5%, mostpreferably, more than 10%.

Preferably, the percentage by volume of the toughening additive in thefinal composite is not more than 50%, more preferably, not more than40%, most preferably, not more than 30%. It is particularly preferredthat the percentage toughening additive by volume in the final compositeis not more than 25%.

The percentage by volume of structural fibres in the fabric ispreferably at least 65%. The minimum value of 65% ensures that there issufficient structural fibres to give the required strength. However theproportion of toughening fibres, that is, the thermoplastic fibres ishigh particularly in comparison to known methods in which thermoplasticis added in particulate form and so the toughening effect iscommensurately much greater than that achieved with those known methods.

Preferably, the melt temperature of the toughening additive is not thesame as the melt temperature of the resin component. It can be between80-350° C., more preferably between 100-250° C., but its final selectionwill depend upon the parameters of the base matrix material.

The ability of the composite to be produced using a low viscosity resinwill implicitly increase the rate at which a mould can be filled.However, the problem of controlling resin cycle times remains. A keyfactor always in thermoplastic resin injection is ensuring that theresin fills the mould and wets the reinforcement totally before it sets.However fill time and setting time are linked and the resin begins toset as soon as it leaves the injection port, and this process continuesthroughout the injection cycle.

In an alternative method, the injection and setting stages of theprocess are separated by incorporating the matrix thermoplastic resin insolid form into the preform. The resin may be in fibrous or particulateform. This has the benefit that the application of heat is all that isrequired to enable the matrix resin to flow and wet the part out andthis will enable even greater convenience in the manufacturing process.

A further preferred feature is the use of a textile veil as part of alaminate by being sandwiched between layers of the structural component.The veil preferably has a greater absorbency rate and the structuralcomponent layer(s) either due to its thinness or the inherent absorbencyor structure of the veil material or a combination of thesecharacteristics. Accordingly, in some embodiments, it is preferred thata veil layer is provided sandwiched between the structural layers andprovides means to increase the rate of filtration of resin into thestructure. Advantageously, by this means, the resin may bepreferentially directed into the centre of thicker structures than hashitherto been possible.

Advantageously, by the use of a fibrous veil, toughness and delaminationsuppression, are achieved with fibre bridging effects. However,preferably, the veil has a toughening additive incorporated therein tofurther enhance toughening of the composite. It is envisaged that theveil may also include thermoplastic fibres as a toughening additive.However, it is particularly preferred that when the veils are made bythe papermaking route, the toughening additive is added in particulateform as this is particularly well adapted for use in the paper-makingprocess.

Preferably, the matrix resin is a thermoplastic material with lowviscosity such as an EMS Chemie Grilamid Polyamide 12.

The fibres may be continuous or discontinuous. If discontinuous, such asare produced by stretch breaking, they will be used in the form of acontinuous yarn formed from the discontinuous fibres.

The structural component fabric may be woven or non-woven and maycomprise a hybrid yarn i.e. structural fibres and toughening fibrestwisted in a hybrid yarn or the fabric may comprise structural yarn andtoughening yarn mixed in a single fabric.

The basic concept of using hybrid yarns can be varied considerably. Itis possible to replace all yarns in a textile with a hybrid yarn, oralternatively to replace a selection. Furthermore a large preform mayconsist of zones of conventional or toughened fabrics according to theneeds of the part. This offers a processing advantage in that a singleresin system can be used for a large part but the properties of thecomposite can differ in terms of toughness, and temperature capabilityfrom place to place—hence making one shot moulding of complex structuresmore feasible.

The properties of the composite can be varied widely by making thepreform of different forms. For example, with a woven fabric the patternin which the structural fibres and the thermoplastic fibres are providedwill have an effect on the overall behaviour of the composite. The useof a structural reinforcement in the form of a textile therefore enablesgreat versatility.

Embodiments of the present invention will now be further described withreference to the accompanying examples and drawings in which:

FIG. 1 a shows a schematic laminar composite in accordance with thepresent invention;

FIG. 1 b shows the upper layer of the laminar composite of FIG. 1 a witha schematic impact region;

FIG. 1 c shows the schematic construction of the upper layer of thelaminar composite of FIG. 1 a;

FIG. 1 d shows an exploded schematic view of yield zone 2 shown in FIG.1 b;

FIG. 2 a shows a hybrid veil sandwiched between two structural layers ina laminate;

FIG. 2 b shows a possible construction for the hybrid veil of FIG. 2 a;

FIG. 2 c shows an alternative construction for the hybrid veil of FIG. 2a;

FIG. 3 shows absorbed energy versus volume fraction x thickness forvarious examples, and

FIGS. 4 to 6 show plots of impact strength as a function of thickness xvolume fraction of fibres for a composite formed from glass fibresalone, FIG. 4, glass fibres and polypropylene fibres, FIG. 5, and glassfibres and polyamide fibres, FIG. 6.

FIG. 1 a shows a composite with a laminar structure of threesuperimposed identical flat rectangular layers: upper layer 3 a; middlelayer b and lower layer c. The internal structure is shown more clearlyby FIG. 1 c which is an explosion of inset 4. The explosion shows eachlayer is formed from a hybrid fabric comprising yarns of structuralfibre, e.g. carbon fibre interspersed with yarns of thermoplastic fibreset in a thermoplastic resin matrix.

FIG. 1 b and FIG. 1 d show schematically the effect of an impact on thesurface of the upper layer 3 a. In particular, FIG. 1 b reveals a seriesof diagonal linear yield zones from the theoretical impact and FIG. 1 dshows an explosion of a linear yield zone 2 and reveals that the yieldzone corresponds to a thermoplastic yarn extending in the compositelayer.

Referring to FIG. 2, this shows a schematic laminar compositeconstruction similar to that of FIG. 1 but with a hybrid veil sandwichedbetween two layers of textile. The sandwiched veil introduces tougheninginto the textile composite. Two alternatives of the veil constructionare shown in FIGS. 2 b and c. FIG. 2 b shows schematically theconstruction of mixed structural and non-structural fibres andthermoplastic powder whereas FIG. 2 c shows a singular construction ofcarbon fibres and thermoplastic powder. In both cases the delaminationresistance and some toughening is provided by fibre bridging between thetextile layers and the fibrous veil. However this is greatly enhanced bythe presence of thermoplastic in the veil layer.

By appropriate design of the interply veil, the resin flow rate acrossthe veil may be enhanced relative to the flow rate across the upper andlower structural layers and thus improve the rate of injected resinimpregnation into the composite.

There will now be described a number of examples of a composite having astructure as illustrated in FIG. 1 but employing a matrix of athermosetting resin. The examples therefore serve to illustrate theeffect of employing thermoplastic fibres as the toughening additive andso are illustrative of the invention even though not fully in accordwith it.

The most dramatic benefits of the thermoplastic fibre tougheningadditive can be seen in the improved impact resistance of thecomposites. This is often illustrated by plotting the absorbed energy inthrough penetration impact tests as a function of volume fraction offibres multiplied by thickness—a combination of parameters that yields amaster curve for conventional composite systems irrespective of matrixtype and detailed fibre orientation (assuming the fibres are arranged ina broadly in-plane isotropy or at worse a 0.90 arrangement). The mastercurve has been found to hold for materials with very different matrices,including brittle cold cure resins and tough thermoplastic matrices,such as polypropylene. Composites with the thermoplastic fibretoughening additive exhibit a surprising increase in toughness as shownby a considerable deviation from the master curve. This is alsoevidenced by greater damage allotment in the impact specimens.

EXAMPLE 1

A composite was prepared from a fabric preform that consisted of glassfibres commingled with polypropylene fibres in a quadriaxial non crimpfabric. The fabric was impregnated with a low viscosity unsaturatedpolyester resin and the laminate was cured at room temperature followedby a post cure at 80° C. in accordance with the resin supplier'sspecification.

The plate was 3 mm thick and the volume fractions of the threecomponents as follows:—

-   -   glass fibres 0.2 v/v;    -   polypropylene fibres 0.2 v/v; and    -   polyester resin 0.6 v/v.

The laminate was subjected to a falling weight impact test to measureits energy absorption. The specific test configuration used producesabsorbed energy results for glass fibre composites that fall in a mastercurve determined by the thickness of the laminate and the volumefraction of fibres. The energy absorbed by the laminate prepared fromthe preform with polypropylene fibres added as toughening agents was 100J.

In contrast, a similar laminate produced from identical polyester resin0.8 v/v but reinforced with a fabric that was totally produced fromglass fibres of a fibre volume fraction of 0.2 v/v and a thickness of 3mm absorbed an average of approximately 40 J. This demonstrates thataddition of the thermoplastic fibres into the preform provides aconsiderable toughness benefit.

EXAMPLE 2

A glass fibre epoxy composite was prepared from a DGEBA epoxy resin(digylcidyl ether of bisphenol-A cured with an amine hardener [ShellEpikote 828 cured with Ciba HY932 aromatic amine]) and a plain weavewoven fabric of E-glass fibres. The fabric occupied approximately 50% byvolume of the composite. A similar composite was prepared with the samelevel of fabric but where the fabric component contained 70% (by volume)E-glass fibres and 30% by volume of a semi-crystalline polymeric fibre,with a crystalline melting temperature of 210° C.

The composites were produced by impregnating the fabrics and laminatingto a thickness of 6 cm thick and cured in an oven set at 190° C.Thermocouples embedded in the centre of the laminate monitored thetemperature rise in the materials as they initially equilibriated to theoven temperature and then experienced further temperature rises due tothe exothermic curing process.

The laminate with just glass fibres exhibited a temperature rise wellbeyond the 190° C. oven temperature which became rapid and reached apeak value of 300° C. at which point significant degradation of theepoxy was observed. The laminate with semi-crystalline thermoplasticfibre also exhibited a temperature rise due to the exothermic cure butonce this temperature reached the crystalline melting temperature of thethermoplastic fibres, the overall temperature rise was halted and theepoxy resin did not noticeably degrade.

EXAMPLE 3

A carbon fibre composite, 3 mm thick, was prepared from a plain weavefabric and an epoxy resin (digylcidyl ether of bisphenol A cured with anamide hardener [Shell Epikote 828 cured with Ciba HY932 aromaticamine]). The fabric contained 70% by volume carbon fibres (Torayca T300)and 30% by volume nylon 6.6 fibres. The fabric was impregnated with theliquid epoxy resin and cured at room temperature for 24 hours followedby a post cure at 100° C. for 4 hours. The cured laminate containedapproximately 50% carbon fibres by volume and 21% of nylon fibres byvolume. The remaining 29% of the composition was cured epoxy resin. Asimilar composite was prepared by impregnating a fabric producedexclusively from carbon fibres. In this case the plain weave carbonfibre occupied 50% of the volume of the composite and the epoxy resinmatrix occupied the remaining 50%.

Both laminates were subjected to excess energy, falling weight impacttests. The laminate comprising just carbon fibres and an epoxy matrixabsorbed 50 J of energy. The laminate with the carbon fibres, nylonfibres and epoxy matrix absorbed 85 J.

EXAMPLES 4 TO 7

Tests have been conducted with a series of medium volume fraction glassfibre composites which exhibit impact toughness (energy absorbed duringdrop weight impact with full penetration) which is enhanced by a factorof 2-3 times by inclusion of thermoplastic fibres in comparison to theunmodified analogues. Tests have also shown a remarkable lack ofsensitivity to notches in open hole tension tests on the same materials.

The impact results of two materials against two control samples areshown in FIG. 3 and Table 1 defines the materials tested.

TABLE 1 Comparison of toughened and non-toughened composite laminates. %Volume Fraction Examples Product Structural Component Structuralcomponents Example 4 (F394) 22-23 Glass/polypropylene/ polyester Example5 (F404) 41 Glass/polyester Example 6 (F384) 22-23 Glass/polypropylene/polyester Example 7 (F389) 25 Glass/polyester

The structural components each comprised about 50:50, glass totoughening additive, by volume.

FIG. 3 shows the impact results for the Examples 4-7 as a plot ofabsorbed energy against thickness x volume of fibres. The impact mastercurve for SMC (sheet moulding composite), GMT's (glass matthermoplastics) and prepreg etc. has been superimposed for comparativepurposes. The absorbed energy for the polypropylene and polyestercontaining composites is significantly improved by comparison withanalogous composites having no toughening additive.

FIGS. 4 to 6 are plots showing impact strength, that is, energy absorbedduring penetration, as a function of thickness x volume fraction offibres. Each plot has data from three different thermoset matrices—twoepoxies and a polyester. The first plot of FIG. 4 shows the resultsachieved when glass fibres alone are used with the volume fraction ofglass fibres in the composite being between 30 to 50%. The second andthird plots of FIGS. 5 and 6 show the results when the portion of theglass fibres is replaced by polypropylene, FIG. 5, and polyamide, FIG.6. The plots demonstrate that the inclusion of the thermoplasticpolymers provide significant benefits in terms of improved impactstrength.

The resins used in the study which produced the plots of FIGS. 4 to 6included an unsaturated isophthalic polyester resin (UP), Crystic 272 (aproduct of Scott Bader plc) and two epoxy systems, EP1 was a cold cureepoxy resin (digylcidyl ether of bisphenol A cured with an amidehardener (Shell Epikote 828 cured with Ciba HY932 aromatic amine) andEP2 was a low single-part, low-viscosity epoxy resin supplied byCytec-Fiberite, Cycom 823, which was cured at 120° C.

The experimental procedure in all of these tests involved the use of aninstrumented falling weight impact test in which a striker equipped witha 20 mm diameter hemispherical tip is allowed to fall onto a platespecimen of the test composite. The composite specimen is a thin plate,typically 3 mm thick, and 60 mm×60 mm in size which is simply supportedon a steel ring with an internal diameter of 40 mm. The striker isdropped from a height of 1 m and has sufficient mass such that thekinetic energy is sufficient for the striker to completely penetrate thespecimen. The test records the forces during the impact event and theenergy absorbed is calculated from the force time record and themeasured velocity of the striker as it impacts the specimen.

As noted in the discussion of the above examples, the use ofthermoplastic fibres incorporated into the resin matrix provides asignificant toughening effect. The thermoplastic fibres give a mechanismfor plastic deformation and yielding which is not possible in anunmodified thermosetting resin. It has now been found that the samemechanism and so toughening effect is produced in a composite having athermoplastic resin matrix which means that the effect is primarilyfibre dominated. This makes it possible to form a composite having athermoplastic resin matrix by a liquid composite moulding technique. Thedesirable qualities of a thermoplastic matrix, including good chemicalresistance and contribution to toughness of the final part, can beobtained without wetting problems. The reason for this is that thepresence of the toughening additives in the form of the thermoplasticfibres in a structural component means that a lower molecular weightthermoplastic can be used for the matrix than would be the case if thematrix was solely to provide the necessary toughness. A reduction inmolecular weight results in a reduction in viscosity and therefore readyimpregnation of the preform.

The fibre thermoplastic may differ from the matrix thermoplastic bymolecular weight with a relatively higher molecular weight thermoplasticbeing used for the fibres and a relatively lower molecular weightthermoplastic for the matrix. This can be achieved by using twodifferent thermoplastics or the same thermoplastic but of two differentmolecular weights. However, it should be noted that the fibrethermoplastic whilst having a higher molecular weight than the matrixthermoplastic does not have such a high molecular weight as to make themodulus such that the fibres are structural. There is no need to usematerials such as Kevlar or other structural thermoplastics. Lowermodulus, and therefore lower cost, thermoplastics will provide thenecessary toughening effect.

The cycle time is key in modem manufacturing. The faster a tool is used,the greater the tool utilisation which reduces tooling costs and soupfront investment in a manufacturing program. Whilst the composite witha thermoplastic matrix and thermoplastic fibre toughening additive ispreferably formed by a liquid composite moulding technique, toolthroughput may be increased if another technique is employed which doesnot involve an injection step. One possibility would be combine thereinforcement component consisting of the structural and non-structuralfibres with further thermoplastic fibres having a melting point lowerthan the thermoplastic fibres of their reinforcement component. Thehybrid preform containing the three types of fibres: structural fibresand thermoplastic fibres of higher and lower melting points would beheated under pressure to cause the low melting point fibres to melt andimpregnate both the structural fibres and the high melting pointthermoplastic toughening fibres. For maximum efficiency, the tool wouldbe heated to a temperature close to the lower melting point prior topositioning the hybrid preform therein.

The low melting point thermoplastic could alternatively be in powderform with identical processing being employed.

1. A composite comprising a structural component and a matrix component,the structural component comprising structural fibres and a tougheningadditive comprising non structural fibres of a first thermoplasticmaterial and the matrix component comprising a second heat-curablethermoplastic material, wherein the structural component is a fabricformed from the structural fibres and the non structural thermoplasticfibres, wherein the fabric comprises non structural thermoplastic fibreswhich are in fibre form in the final composite, and wherein the firstand second thermoplastic materials are different.
 2. A composite asclaimed in claim 1 wherein the first and second thermoplastic materialsdiffer as to their molecular weight.
 3. A composite as claimed in eitherclaim 1 or claim 2 wherein the first and second thermoplastic materialsare dissimilar.
 4. A composite as claimed in any preceding claim whereinthe matrix component is a low viscosity thermoplastic material.
 5. Acomposite as claimed in any preceding claim wherein at least some of thethermoplastic fibres are semi-crystalline.
 6. A composite as claimed inany preceding claim wherein the percentage by volume of the tougheningadditive is more than 2% but less than 50%.
 7. A composite as claimed inany preceding claim wherein the volume of the toughening additive ismore than 5% but less than 40%.
 8. A composite as claimed in anypreceding claim wherein the volume of the toughening additive is morethan 10% but less than 30%.
 9. A composite as claimed in any precedingclaim wherein the structural component is provided in the form of aplurality of layers of fabric and at least one veil is provided betweena pair of adjacent layers, the veil comprising a thin layer of woven orunwoven material.
 10. A composite as claimed in any preceding claimwherein the volume fraction of the structural fibres in the fabric is atleast 65%.
 11. A composite as claimed in any preceding claim wherein thestructural and/or non structural fibres are continuous or discontinuous.12. A composite as claimed in any preceding claim wherein the fabriccomprises a hybrid yarn of twisted structural fibres and thermoplasticfibres or yarn of structural fibres and yarn of thermoplastic fibres.13. A method of making a composite comprising forming a fabric fromstructural fibres and non structural fibres of a first thermoplasticmaterial to provide a structural component, injecting a liquid resincomprising a second thermoplastic material into the structural componentto provide a matrix component and setting the matrix component, whereinthe first and second thermoplastic materials are different and whereinthe liquid resin is injected at a temperature such that the finalcomposite includes non structural thermoplastic fibres in fibre form.14. A method as claimed in claim 13 wherein the fabric is provided inlayers and a veil is provided between at least one adjacent pair oflayers prior to addition of the second thermoplastic material, the veilcomprising a thin layer of woven or non-woven material.
 15. A method asclaimed in claim 14 comprising distributing binder material on or in theveil.
 16. A method as claimed in any one of claims 13 to 15 wherein theresin injection process is resin transfer moulding or composite resininjection moulding.